This invention relates to optical information medium such as read-only optical disks and optical recording disks, and a method for testing storage stability of an optical information medium having a resin layer.
To record and store a vast quantity of information as typified by moving image information, advanced optical disks such as read-only optical disks and optical recording disks are required to increase their recording density for increasing the capacity. To meet such a demand, engineers have been engaged in the research and development works targeting a higher recording density.
One such approach relating to digital versatile disks (DVD) is to shorten the wavelength of a recording/reading laser beam and increase the numerical aperture (NA) of a recording/reading optical system objective lens, thereby reducing the spot diameter of the recording/reading laser beam. As compared with CD, DVD is successful in achieving a recording capacity of 6 to 8 folds (typically 4.7 GB/side) by changing the recording/reading wavelength from 780 nm to 650 nm and the NA from 0.45 to 0.6.
For long-term recording of moving images of quality, an attempt was recently made to achieve a recording capacity of at least 4 folds of that of DVD, i.e., at least 20 GB/side, by reducing the recording/reading wavelength to about 400 nm and increasing the NA of the objective lens to about 0.85.
Increasing the NA, however, leads to a reduced tilt margin. The tilt margin is a permissible tilt of an optical recording medium relative to an optical system, which depends on the NA. The tilt margin is in proportion to
xcex/(txc2x7NA3)
wherein xcex denotes the wavelength of recording/reading beam and xe2x80x9ctxe2x80x9d denotes the thickness of a transparent substrate the recording/reading beam enters. If the optical disk is inclined or tilted relative to the laser beam, a wavefront aberration (or coma) occurs. The coefficient of wavefront aberration is represented by
(xc2xd)xc2x7txc2x7{n2xc2x7sin xcex8xc2x7cos xcex8}xc2x7NA3/(n2xe2x88x92sin2xcex8)xe2x88x925/2
wherein n denotes the refractive index of the substrate and xcex8 is a tilt angle. It is appreciated from these formulae that the tilt margin may be increased and the occurrence of comatic aberration be suppressed by reducing the thickness xe2x80x9ctxe2x80x9d of the substrate. In fact, the DVD design is such that a tilt margin is secured by reducing the thickness of the substrate to about one half (about 0.6 mm) of the thickness (about 1.2 mm) of the CD substrate.
To record moving images of better quality for a longer period of time, there has been proposed a structure allowing for use of a thinner substrate. In this structure, a substrate of an ordinary thickness is used as a supporting substrate for maintaining rigidity, pits or a recording layer is formed on the surface of the supporting substrate, and a light-transmitting layer of about 100 xcexcm thick is formed thereon as a thin substrate. Recording/reading beam reaches the pits or recording layer through the light-transmitting layer. This structure can achieve a higher recording density due to a higher NA because the substrate can be made extremely thin as compared with the prior art. One typical optical disk having such structure is disclosed in JP-A 289489/1998. The disk is described therein as having a light-transmitting layer of a photo-curable resin.
When the light-transmitting layer is formed of photo-curable resins such as UV-curable resins, however, the optical disk can deflect due to shrinkage upon curing. Deflection can also occur when the optical disk is stored in a hot humid environment. Once the optical disk deflects, loading of the disk in the optical disk drive may become difficult, and once the optical disk is deflected with twisting, axial runout of the optical disk increases and frequent errors occur upon reading, and excessive deflection can cause the optical disk to be unreadable. In particular, when the medium is recorded/read using a laser beam with reduced beam spot diameter and at a high linear velocity, focus servo becomes considerably unstable due to the increase in axial runout acceleration.
By reducing the recording/reading wavelength, increasing the NA of the objective lens to reduce the beam spot diameter, and increasing the linear velocity during recording and reading, there can be achieved a significant improvement in data transfer rate. Even a data transfer rate of 100 Mbps or higher is possible. With the start of the satellite digital broadcasting system at the end of 2000, image information of high quality is now delivered to home. A remarkable improvement in data transfer rate is thus demanded for recording such image information. However, the focusing servo stability must be improved before the data transfer rate can be increased.
The inventors of the present invention have also found through their investigation that, when a surface layer of high hardness is formed on the light-transmitting layer to improve the scratch resistance, the medium undergoes increased deflection with twisting during hot humid storage to invite considerable increase in the axial runout as well as cracks and fractures of the surface layer. It was also found that such cracks and fractures are more significant when the medium is subjected to a thermal shock test wherein the medium is subjected to alternate high temperature storage and low temperature storage compared to static storage wherein the medium is stored under high temperature or high humid conditions.
An object of the invention is to provide an optical information medium comprising a supporting substrate, an information recording layer thereon, and a light-transmitting layer thereon wherein a recording or reading laser beam enters the medium through the light-transmitting layer, in which recording/reading characteristics are improved even when the beam spot of a laser beam has a small diameter and the linear velocity is high without compromising the function of the light-transmitting layer of protecting the information recording layer. Another object is to improve storage stability of such medium.
Such objects are attained by the present invention as described in (1) to (8), below.
(1) An optical information medium comprising a supporting substrate, an information recording layer thereon, and a light-transmitting layer on the information recording layer, wherein a recording or reading laser beam enters the medium through the light-transmitting layer, wherein
said light-transmitting layer includes at least one resin layer, and said light-transmitting layer has a tensile yield stress of 20 to 100 MPa and a tensile strain at yield of 0.1 to 15%.
(2) An optical information medium according the above (1) wherein said light-transmitting layer comprises at least one resin-containing inner layer and one surface layer which is harder than the inner layer by at least 1 unit in pencil hardness, and said surface layer constitutes a surface of the medium.
(3) An optical information medium according to the above (2) wherein said inner layer comprises a resin sheet or a coating of a resin containing a radiation-curable resin.
(4) An optical information medium according any one of the above (1) to (3) wherein said light-transmitting layer has a thickness of 30 to 200 xcexcm.
(5) An optical information medium according to any one of the above (1) to (4) wherein said light-transmitting layer includes at least two annular resin layers and at least one of said resin layers is a resin coating, and said resin coating has an inner diameter larger than that of its adjacent resin layer on the side of the supporting substrate.
(6) A method for testing storage stability of an optical information medium comprising a substrate, an information recording layer thereon, and at least one resin layer on the information recording layer, wherein the method comprises the steps of
exposing the optical information medium to at least thirty cycles each comprising a high temperature environment and a low temperature environment with the temperature difference of at least 70xc2x0 C., and thereafter inspecting mechanical precision of the optical information medium and cracks and peeling of said resin layer.
(7) A method for testing an optical information medium according to the above (6) wherein the medium has at lest two resin layers.
(8) A method for testing an optical information medium according to the above (6) or (7) adapted for use with the optical information medium of any one of the above (1) to (5).